1. Field of the Invention
This invention pertains generally to the field of cameras and more particularly to cameras having adjustable lenses.
2. Description of the Prior Art
Present quality cameras all utilize lenses which are adjustable with respect to focus or distance setting, aperture setting, and sometimes focal length. In addition, such cameras have a nominal image receiving surface (film plane in the case of a film camera, or surface of other substance capable of recording the image as in a television camera), and a focusing analog of the nominal image receiving surface for indicating or determining correct focus. As used herein, "focusing analog" means any apparatus or object used in connection with a camera whereby an approximation or analog of the image which will be projected by the lens onto the image receiving surface can be visually or electronically observed prior to shutter release. This is typically accomplished in a single lens reflex camera by utilizing a flip-up mirror which directs the image from the lens onto a focusing screen which is then optically transmitted to the eye of the photographer. In theory, the focusing screen and the nominal film plane of the camera are optically equidistant from the lens so that the image on the focusing screen is exactly the image that will be presented to the film. Thus the photographer can manipulate the focus adjustment on the lens assembly as desired until the object appears in focus to him. Additionally, most single lens reflex cameras provide a focusing aid at the center of the focusing screen.
Other types of cameras utilize focusing analogs which does not perceive the image through the actual lens of the camera. Examples of these cameras are the double lens reflex camera and the coupled rangefinder camera. The coupled rangefinder camera has utilized various ranging devices such spaced mirrors or more sophisticated means to converge upon an object and provide information as to the distance from the lens to the object. The rangefinder is coupled to the lens in a predetermined relationship such that when the rangefinder shows that focus has been achieved, theoretically the image of the object should be focused by the lens upon the film within the camera. Studio cameras may simply provide a focusing screen which is positioned within the camera at the nominal film plane during focusing of the lens after which it is replaced by the film prior to shutter release.
With all practical lens systems used in cameras, the actual "image surface" (that surface which would intercept the best focus of the image at an infinitude of points within the image) generated by the lens for any setting of the lens is not a perfect plane and thus cannot be made to precisely coincide with any nominal film plane. This is so because of residual aberrations such as spherical aberration and astigmatism which occur in any practical lens system, and is well known to camera and lens manufacturers. As a result of these departures from perfect lens systems, the actual image surface will be curved and buckled, and frequently somewhat skewed at an angle with respect to the nominal plane of the film because of manufacturing defects. The film itself may, of course, depart in position from the nominal film plane because of ripples in the film and twisting within the film holder, but the nominal film surface is normally assumed to be a plane.
One means of determining the shape of the image surface is to perform through-focus contrast testing, in the manner explained below, to develop a three-dimensional picture of the image surface of the lens. It is found that the shape and position of the image surface varies both with the focus adjustment, the aperture setting and the focal length of the lens. Of course, the shape and variability of the image surface will be different from lens to lens. Typically, the manufacturer of the camera will attempt to fix the position of the nominal film plane with respect to the lens such that the film plane is fit to some average focus and aperture setting of the lens. Where the actual focus and aperture setting varies considerably from the average set by the manufacturer, loss of focus will occur at some portions of the film, particularly in the peripheral area surrounding the center of the picture. Accordingly, even the highest quality present cameras do not achieve optimum focusing of the image on the image receiving surface for most lens adjustments, and produce pictures in which substantial areas exhibit substantial loss of contrast and sharp focus.